Coherence between teleseismic tomography and long-wavelength features of the gravity and magnetic fields of southeastern Australia

Abstract

During 2007 the NSW Department of Primary Industries contracted the Australian National Research Facility for Earth Sounding (ANSIR) to conduct the SEAL2 (South East Australia Linkage experiment, part 2) teleseismic program in southwestern New South Wales, to supplement earlier teleseismic acquisition in Victoria and southeastern South Australia (Figure 1). Interpretation of the results of the earlier SEAL1 program (Rawlinson et al., 2006), acquired with a seismic array in the southern Murray basin, appeared to conflict with recent interpretations of upper-crustal tectonic trends inferred primarily from aeromagnetic surveys in southwestern NSW (Hallett et al., 2005), which suggested that the Stawell Zone of western Victoria continued northwards into NSW. Instead, the SEAL1 tomographic model suggested a break in upper mantle velocities near the state border; the model implied that the Stawell Zone in Victoria is underlain by high vp upper mantle, consistent with lithospheric basement of Proterozoic age, while the putative extension of the Stawell Zone into NSW is underlain by low vp mantle, corresponding to Palaeozoic basement. Analysis of the SEAL2 dataset included a synthesis of all the southeast Australian teleseismic data into a single tomographic model of the upper mantle (Rawlinson, 2008). This model complicated the interpretation of the Stawell Zone further, by indicating a boundary between high vp ?Proterozoic? and low vp ?Phanerozoic? upper mantle cutting obliquely across the boundary between the Stawell and Bendigo zones in Victoria. In addition, two lobes of high vp upper mantle east of 145°E lie below Phanerozoic upper crust of the Lachlan Orogen, and lack any apparent relationship to internal tectonic divisions within the Lachlan Orogen. This paper investigates, and seeks to resolve, the apparent discrepancy between the tomographic upper mantle models on the one hand, and the combination of mappable surface geology and interpreted upper-crustal geophysics on the other, by comparing both with geophysical and geological evidence which arises from sources located in the middle to lower crust. Potential field anomalies with wavelengths greater than about 20 km should reveal middle crustal features, although the ability to discriminate them is compromised by the presence of broad anomalous sources (granitoids) in the upper crust, and, for the magnetic field, by suppression of long wavelengths during grid merging.